(1) Field of the Invention
The present invention relates to a process for dicing a preform made of an oxide single crystal and a process for producing functional devices.
(2) Related Art Statement
The traveling wave optical modulator in which an oxide single crystal such as lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3) or quartz is applied to an optical waveguide has excellent characteristics, and can realize a high speed modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent as ferroelectric materials, and are advantageous in that they possess high electro-optic coefficients, and can control light within a short optical path. Further, research have started to form SHG elements (second harmonic generation elements) on oxide single crystal substrates.
On the other hand, in addition to such optical devices, research have been also begun to form piezoelectric devices such as piezoelectric oscillators from such single crystals.
n both optical devices and piezoelectric devices it is beneficial that a number of devices be formed on a surface of a wafer, and that the resulting wafer is then diced into a number of individual devices.
Heretofore, a cutting method using an outer peripheral cutting type, extremely thin grinding stone has been known to dice the oxide single crystal wafers ("Latest Cutting Technique Handbook" edited by Latest Cutting Technique Handbook Editing Committee, publisher: Industrial Technical Service Center Co., Ltd., pp 70-73). According to this method, fine diamond abrasive grains and a 20-40 .mu.m and 20-40 thick resinoid or metal-bonded grinding stone are used. According to this method, if a grinding stone having a small grit size is used, extremely high precision, machining can be performed and the state of cut surfaces is very good.
However, when dicing the oxide single crystal wafers according to this method the following problems arise.
(1) In order to prevent rise in temperature with frictional heat during cutting, the wafer needs to be cut while cooling water is being splashed onto the wafer. Since the devices are contaminated by this cooling water, a post washing step is required.
(2) Unless a considerably high mechanical accuracy is exercised during a feed stage and with respect to a spindle for moving the wafer, chipping or cracking occurs in cut pieces during only slight movement of the grinding stone. Moreover, the wafer needs to be fixed at a considerably high accuracy.
(3) Since the grinding stone has already been worn, the stone has a small grit size and, the grinding stone needs to be adjusted due to the change in the shape of the cutting surface of the grinding stone so that the depth of a groove formed may be identical during each machining.
(4) In addition to the above problems, a pre-step such as surface-coating and steps of peeling the surface coating on the wafer and washing the cut pieces, even if the cutting speed is highly raised. Consequently, a total time required for the dicing becomes longer. Furthermore, heating is necessary on surface-coating, resin-fixing and drying after the washing. Since the oxide single crystal is weak against heat impact, a very careful attention must be paid during the heat treatment and to prevent uneven characteristics of the resulting products.
In order to avoid the above problems, the present inventors cut oxide single crystal wafers through cleavage by way of experiments. Ionic crystals such as LiF and NaCl can be cleaved and cut along their specific crystalline faces by hitting them with a knife edge ("Latest Cutting Technique Handbook" published by Industrial Technical Service Center Co., Ltd., pp 258-259). Further, a GaAs single crystal substrate and an InP single crystal substrate for semiconductor laser can be also cut by cleavage, and resonator mirrors can simultaneously produced. Since the cleaved surface of the crystal having good cleavage property is flat and smooth as viewed from the atomic level, it need not be optical polished.
However, the oxide single crystal is unlikely to be cleaved, so that it cannot be practically used.
With respect to the silicon wafer, a inscribe groove is formed on the silicone wafer by using a diamond cutter, and the wafer is cleaved along this inscribe groove. Such a method is used for glass, too.
However, it is more difficult to machine the oxide single crystals, for example as compared with silicon, and even if an inscribe line is formed on the surface of an oxide single crystal wafer and then the wafer is hit, the wafer is likely to be cracked in every direction rather than in a given direction from the inscribe groove. Consequently, the cut surface has many uneven portions. In addition, since the oxide single crystal is generally so hard that it may be difficult to form an inscribe line thereon, this method renders the wafer difficult to be cut, which increases the occurrance of unacceptable products and decreases the yield of the devices.